Standard installation procedures for subsurface wells (sometimes referred to herein simply as ‘wells’) have been established in the environmental industry. In the early years following the establishment of the United States EPA program in the US (ca. 1980), many monitoring wells were of a 4-inch diameter or greater for the purpose of accommodating readily available fluid pumps that were used in the water resources business, for example, with these pumps being of a 3-inch diameter and greater. In the mid to late 1980s, smaller diameter pumps were developed specifically for groundwater monitoring applications. As a result, the environmental industry found it possible to reduce monitoring well installation costs by installing 2-inch diameter monitoring wells to accommodate these smaller diameter fluid purging and sampling pumps. Drilling machines that were used for the 2-inch and greater diameter wells were typically auger, rotary or casing drive based technologies—such as hollow stem auger, mud rotary and air rotary, air rotary casing hammer, dual wall percussion and even sonic. These drilling technologies often remove large quantities of soil, rock, and formation fluid to advance a well bore. The costs associated with drilling, containerizing and disposing of these materials can be significant.
Given the expense of using these types of large drilling machines, direct push drilling technology emerged as a viable technology in the early 1990s—making it possible to reduce costs even further for shallow drilling projects typically ranging between 10 to 60 feet below ground surface (and even deeper with cone penetrometer (CPT) machines). One feature of the direct push drilling method was the minimization of drill cuttings and fluids by means of simply displacing the unconsolidated sediment to the side of a drive cone or point during borehole advancement, as opposed to removing the cuttings and fluids from the borehole. A key requirement in accomplishing this procedure was to reduce the diameter of the drive cone and drive rod to diameters typically less than 1.5 to 2-inches in order to reduce frictional surface area which is critical for direct push borehole advancement. As a result of the direct push technology, relatively small diameter monitoring wells could be installed to shallow depths at significant cost savings compared to 2-inch and 4-inch monitoring wells installed with more traditional drilling technologies (described above).
Fluid monitoring wells consist of a riser pipe with an attached fluid inlet structure at a bottom end of the riser pipe, and are normally of a diameter of at least 2 inches. They are installed in the ground to a depth of a fluid to be sampled and with the fluid inlet structure being of an appropriate length. Once the well structure is in place with the desired configuration, fluid from one zone flows into the riser pipe and rises to an equilibrium point within the pipe. Fluid is then sampled from within the riser pipe using various methods. Unfortunately, a problem with the above-described drilling technologies is that there is no isolation of well bore fluids between the riser pipe and fluid inlet structure of the fluid monitoring well, regardless of diameter.
With conventional technology, it is difficult or impossible to cost-effectively and properly isolate the standing fluid in the riser pipe from the desired fluid in the fluid inlet structure. Therefore, it is entirely possible for the stagnant and possibly non-representative fluid in the riser pipe to mix with the fluid in the fluid inlet structure during purging and sampling. As a result, the collected fluid samples may be altered or biased to provide a non-representative result.
In an effort to reduce the negative impact to these fluid samples and increase the likelihood of relatively representative results, environmental regulations within the fluid monitoring industry require certain amounts of fluid be purged from the riser pipe prior to sampling to remove stagnant standing fluid and/or fluid that is non-representative. Many branches of state and local environmental agencies still require that at least 3 to 5 wet casing volumes be removed from the well structure as a means of eliminating all of the stagnant and non-representative fluid from the fluid inlet structure and riser pipe zones. Hence, there is significant fluid drawdown inside the well to facilitate this process. As stagnant and/or non representative fluid is removed, new fluid is drawn into the riser pipe from the fluid inlet structure. In theory, the intent of this process is to increase the likelihood that the fluid samples taken will statistically reflect actual fluid conditions. The downside to using this procedure, however, is that it is necessary to remove (purge) substantial quantities of fluid at a significant cost.
Many state and local agencies now allow a procedure called “low-flow sampling” as a common practice for the purpose of reducing the amount of fluid purged when using 3 to 5 wet casing volumes. Low-flow sampling requires that the fluid within the riser pipe not be drawn down significantly during the sampling event. Therefore, the recharge rate of fluid into the riser pipe from the intake area must be nearly equal to the rate of fluid discharged during purging and sampling. This can require monitoring of actual drawdown during sampling by means of an electrical or fiber optic transducer inserted into the well to detect changes in fluid level.